Elms do collapse, but not often (6)
I believe the answer is:
seldom
'not often' is the definition.
('seldom' is the opposite to 'often')
'elms do collapse' is the wordplay.
'collapse' indicates an anagram.
'elms'+'do'='elmsdo'
'elmsdo' with letters rearranged gives 'SELDOM'.
'but' is the link.
(Other definitions for seldom that I've seen before include "Not often, infrequently" , "on rare occasions" , "Infrequent" , "Alluvium" , "Once in a blue moon" .)